1. Field of the Invention
The present invention relates to reclosable plastic bags and packages of the type in which food products, such as chips and cereal, and other goods are packaged for sale to consumers. More particularly, the present invention relates to a reclosable zipper strip for use in transverse-zippered reclosable plastic bags made on form-fill-seal (FFS) machines.
2. Description of the Prior Art
The present invention relates to improvements in the package-making art and may be practiced in the manufacture of thermoplastic bags and packages of the kind that may be used for various consumer products, but which are particularly useful for food products which must be kept in moisture and air-tight packages, free from leakage until initially opened for access to the product contents, which packages are then reclosable by zipper means to protect any remainder of the product therein.
The prior art is fairly well-developed, but nevertheless remains susceptible to improvement contributing to increased efficiency and cost effectiveness.
In particular the present invention relates to the area of reclosable packaging known as the transverse zipper. When making a bag with a transverse zipper, the zipper is attached transverse to the longitudinal axis of the material used to make the bag, as opposed to being attached to the material parallel to the longitudinal axis. A method and apparatus for making reclosable plastic bags with a transverse zipper on a FFS machine is disclosed in U.S. Pat. No. 4,909,017.
Prior to the introduction of the transverse zipper to the reclosable packaging field, reclosable plastic bags made on FFS machines were typically made with a longitudinal zipper, i.e. a zipper parallel to the longitudinal axis of the thermoplastic film used to make the bags. However, there are two primary problems with the longitudinal zipper. First, there is a problem in attaining satisfactory sealing of the bags against leakage since the transverse, or side, sealing bars of the FFS machine must flatten and seal the zipper at the same time they are sealing the thermoplastic film from which the packages are being made. The difficulty with which this sealing is consistently and successfully achieved is reflected by the high occurrence of leaking packages.
Second, the length of reclosable bags made on FFS machines when the zipper is attached parallel to the longitudinal axis of the thermoplastic film is limited to the diameter of the filling tube of the FFS machine. Thus, generally bags of this type are wider than they are long. While such bags are suitable for certain products where shorter bags are desirable, such as cheese and chicken parts, these bags are not suitable for applications in which longer bags are desirable, for example chips and other snack foods.
Among the approaches taken to solve these problems has been the substitution of a transverse zipper for the longitudinal zipper. When a transverse zipper is provided, the transverse sealing bars associated with the FFS machine do not flatten the zipper during formation of the top and bottom seals of the package since the transverse sealing bars may seal the zipper to the thermoplastic sheet material transversely thereacross without contacting the zipper. In addition, when a transverse zipper is used the length of the packages made on the FFS machine can be varied and is not limited to the diameter of the filling tube.
The present invention relates to a particular type of zipper strip which may be used in transverse zipper applications, a method for attaching the zipper strip to the thermoplastic film, and a method for making reclosable bags using the zipper-equipped thermoplastic film.
A typical zipper strip used in transverse zipper applications has interlocked male profile and female profiles. Each profile has an interlocking member and a laterally extending web defining a leading flange and a trailing flange on either side of the interlocking member. The zipper strip is initially secured to the thermoplastic film by sealing the leading flange of one of the profiles thereto. This allows the film to carry the zipper strip into the FFS machine where the bag is formed and the final seals are made. The problem with this approach, however, is that the leading flange which is not secured to the film has a tendency to flare out as the film enters the FFS machine and folds around the filling tube. The result of this flaring is that the unsecured leading flange has a tendency either to jam the FFS machine or to become caught on the machine and thereby cause the profiles to separate. While the zipper strip can be made without the second leading flange and thereby avoid this problem, the absence of this second leading flange and its subsequent attachment to the film can result in poor zipper opening mechanics and a weak seal between the zipper strip and the bag when the bag is opened for the first time by pinching the front and rear panels of the bag and pulling them apart (the current practice for opening snack packages). A four-flange seal, that is the male and female leading and trailing flanges being sealed to the bag walls, is the best and most secure method of attaching the zipper.
It is therefore an object of the present invention to provide a zipper strip for use in transverse-zippered reclosable plastic bags having two leading flanges which overcomes the aforementioned flaring problem. Another object of the present invention is to provide a method for attaching the zipper strip to the thermoplastic film such that there is no danger of jamming the FFS machine or separating the zipper as the transverse zipper-equipped film enters the FFS machine. Yet another object of the present invention is to provide a method of making reclosable bags using the transverse zipper-equipped film. Yet another object of the present invention is to provide a bag which utilizes the zipper strip.